The present invention relates to a voltage-controlled oscillator circuit used in a phase-locked loop or the like and, more particularly, to such a circuit intended for stabilization of its free-running frequency, and its application to reading a floppy disk. A fairly large number of voltage-controlled oscillator circuits have already been disclosed. As an example, a voltage-controlled oscillator circuit for use in a complementary metal-oxide integrated circuit on one chip used for a phase-locked loop (PLL), is shown in FIG. 3, wherein the electric current flowing into a capacitor C.sub.1 is controlled by a control voltage applied to the gate of an N-channel transistor N.sub.1 to control the frequency of oscillation. Resistors R.sub.1 and R.sub.2 determine the control voltage sensitivity coefficient and the free-running frequency, respectively.
Another example as disclosed in Japanese Patent Publication No. 86509/81 is shown is FIG. 4, wherein current flowing into ring oscillators is controlled by the gate voltage to transistors T.sub.41 -T.sub.46 connected to the sources to control the frequency of oscillation. This circuit has advantages over the circuit of FIG. 3 in dispensing with externally attached parts and requiring less electric operating current and less space to mount components, but it is difficult to obtain good accuracy and stability. Also, it cannot be said that the circuit of FIG. 4 has a sufficient stability.
Generally, what must be regulated in a voltage-controlled oscillator circuit are the free-running frequency and the voltage control sensitivity coefficient. The free-running frequency is defined as the frequency of oscillation when the control voltage applied to the control terminal of a voltage-controlled oscillator circuit is at a reference level. The reference level is usually set at a central value of the controllable range of input voltage. For instance, in the case of a CMOS IC, the reference level is set to half of the power supply voltage. If it is assumed that the difference between the control voltage V.sub.c and the reference voltage level V.sub.s is .DELTA.V.sub.c, then EQU .DELTA.V.sub.c =V.sub.c -V.sub.s ( 1)
Thus, it may also be said that the free-running frequency is the frequency of oscillation when .DELTA.V.sub.c =0. The voltage control sensitivity coefficient K.sub.v is defined by EQU f.sub.o =f.sub.c +K.sub.v .multidot..DELTA.V (2)
where f.sub.c is the free-running frequency and f.sub.o is the frequency of oscillation of that voltage-controlled oscillator circuit.
Drift in the free-running frequency in a PLL has an adverse effect on the loop because it introduces drift in the capture range of the system. Also, dispersion in the frequency f.sub.c due to dispersion in constants of circuit components is so large that it cannot be neglected. Therefore, high-precision components have been conventionally used at the sacrifice of curtailment of cost, or otherwise adjustments or alignments using semifixed resistors or semifixed capacitors have been required after assembly. Also, drift in the voltage in the voltage control sensitivity coefficient K.sub.v in a PLL has an adverse effect on the loop because it results in drift in the response velocity of the system.
These drifts are attributed to change in the ambient temperature, fluctuation in the power supply voltage, aging of component constants, etc. Especially, the free-running frequency f.sub.c is affected greatly by these factors. On the other hand, variation in the coefficient K.sub.v can be reduced by providing sufficient relative tracking characteristics employing semiconductor and integrated circuit techniques such that the value K.sub.v depends on relative accuracy among components of the circuit, although absolute accuracy of the constants of elements is not obtained.
It is possible even with prior art techniques to fabricate the components of the variable-frequency oscillator on one chip by attaching discrete components to the outside surface of the chip, and fabricating the other components in a semiconductor integrated circuit format. However, features inherent in semiconductor integrated circuitry cannot be fully achieved from such construction. Specifically, the amount of space to mount components cannot be made small. Also, a large number of junction points do not assure good reliability and operation. The number of steps for mounting components and the cost of fabricating this circuit cannot be reduced. Thus, a considerable limitation is imposed on the design of such a chip for a semiconductor integrated circuit.
In particular, when connections are made to externally attach components from elements inside of the semiconductor integrated circuit chip, a considerably larger chip area is needed as compared with a construction where no connections are required, because pads for connection, and transistors in the outer buffer circuits are several ten-times as large as those of a device eliminating external connections. Furthermore, the cost of packaging for the integrated circuit is increased.
Another disadvantage of the prior art circuitry is that adjustments are necessary after assembly. This results from the fact that stable and precise variable frequency oscillators are not available.
What is needed is a variable frequency oscillator which is highly stable and produces substantially by integrated circuit techniques with small size, low cost and no necessity for adjustment after assembly.